[gdb/testsuite] Add debug prints in Term::wait_for

[gdb/testsuite] Add debug prints in Term::wait_for

The semantics of wait_for are non-trivial, and a bit hard to understand
sometimes.

Add some debug prints in wait_for that make it clear:
- what regexps we're trying to match,
- what strings we compare to the regexps, and
- whether there's a match or mismatch.

I've added this ad-hoc a couple of times, and it seems that it's worth having
readily available.

The debug prints are enabled by adding DEBUG_TUI_MATCHING=1 to the
RUNTESTFLAGS:
...
$ make check RUNTESTFLAGS="gdb.tui/empty.exp DEBUG_TUI_MATCHING=1"
...

Tested on x86_64-linux.